This invention relates to RF load and source pull testing of medium and high power RF transistors and amplifiers using remotely controlled electro-mechanical impedance tuners.
Modern design of low noise or high power RF amplifiers and mixers, used in various communication systems, requires accurate knowledge of the active device's (microwave transistor's) characteristics. In such circuits, it is insufficient for the transistors, which operate as highly non-linear devices, close to power saturation, to be described using linear or non-linear numeric models.
A popular method for testing and characterizing such microwave components (transistors) in the non-linear region of operation is “load pull” or “source pull”. Load/source pull is a measurement technique employing microwave impedance tuners and other microwave test equipment (FIG. 1), such as signal source (1), input and output tuner (2, 4), power meter (5) and test fixture (3) which includes the DUT. The tuners and equipment are controlled by a computer (6) via digital cables (7, 8 and 9). The microwave impedance tuners are devices which allow manipulating the RF impedance presented to the Device Under Test (DUT, or transistor) to test (see ref. 1); this document refers hence to “impedance tuners”, in order to make a clear distinction to “tuned receivers (radios)”, popularly called elsewhere also “tuners” because of the included tuning circuits (see ref. 2).
Electro-mechanical impedance tuners (FIG. 2) in the microwave frequency range between 100 MHz and 60 GHz are using the slide-screw concept and include a slabline (24) with a test port (25) and an idle port (26), a center conductor (23) (see also FIG. 3) and a number of mobile carriages (28) which carry a motor (20) each, a vertical axis (21) which controls the vertical position (29) of a reflective probe (22). The carriages are moved horizontally (217) by additional motors (not shown) and gear (27). The signal enters into one port (25) and exits from the other (26). In load pull the test port is the one where the signal enters, in source pull the test port is the one where the signal exits. The entire mechanism is, typically, integrated in a solid housing (215) since mechanical precision is of highest importance.
The typical configuration of the reflective probe inside the slabline is shown in FIG. 3: a number of such parallel conductive (preferably metallic) reflective tuning elements (31) also called “tuning” probes or “slugs”, are inserted into the slotted transmission airline (34) and are coupled capacitively with the center conductor (33) to an adjustable degree, depending from very weak (when the probe is withdrawn, top position) to very strong (when the probe is very close (within electric discharge—or Corona distance “D”, bottom position) to the center conductor; it must be pointed out that capacitive “tuning” probes are different from “sampling” probes, which are loosely coupled with the center conductor; when the tuning probes move vertically (36) between a “top position” and a “bottom position” and approach the center conductor (33) of the slabline (34) and moved along the axis (35) of the slabline, they alter the amplitude and phase of the reflection factors seen at the slabline ports, hereby covering parts or the totality of the Smith chart (the normalized reflection factor area). The relation between reflection factor and impedance is given by GAMMA=(Z−Zo)/(Z+Zo), where Z is the complex impedance Z=R+jX and Zo is the characteristic impedance. A typical value used for Zo is 50 Ohms (see ref. 3). When the tuner is terminated at one ports with Zo (50 Ohm), GAMMA at the other port is equal to the first element of the slabline s-parameter matrix: GAMMA=S11 when terminated at port 2 (the right port) or GAMMA=S22 when terminated at port 1 (the left port).
Up to now such metallic tuning probes (slugs, FIGS. 4A, and 4B), have been made in a cubical form (45) with a concave bottom (402) which allows capturing, when approaching the center conductor (401) (49), the electric field, which is concentrated in the closest space between the center conductor and the ground planes (47) of the slabline. This field capturing allows creating high and controllable reflection factors. Contact of the probes with the sidewalls (40, 44) is critical. It can be either capacitive (44, FIG. 4b) or galvanic (40, FIG. 4a). If the contact is capacitive, the surface of the probes and/or the sidewalls of the slabline must be electrically insulated. This can be done using chemical process such as anodization (see ref. 5). Capacitive ground contact means extreme requirement in sidewall planarity and straightness to keep the quasi-sliding contact constant for the whole length and depth of the slabline as the probe moves into and along the slabline. Galvanic contact is safer, more repeatable and less vibration sensitive, but requires a spring mechanism (41) to allow for constant pressure of the probe on the sidewalls (46) (FIG. 4A). This is made by machining a hole (42) and slot (48) in the probe body. The probe is held by the axis via a pin (43).